Radioactive materials are often detected and identified by measuring gamma-rays emitted from the materials. The energy of gamma-rays is specific to that particular material and acts as a “finger print” to identify the material. A high resolution detector typically has better sensitivity and identification capability. In the following discussion, the terms “gamma rays”, “radiation”, and “photons” are used interchangeably.
The ability to detect gamma rays is a vital tool for many areas of research. Gamma-ray detectors allow scientists to study celestial phenomena and diagnose medical diseases, and they have been used to determine the yield in an underground nuclear test. Today, these detectors are an important tool for homeland security, helping the nation confront new security challenges. Government agencies need detectors for the scenarios in which a terrorist might use radioactive materials to fashion a destructive device targeted against civilians, structures, or national events. To better detect and prevent terrorist attacks, the Department of Homeland Security (DHS) is funding projects to develop a suite of detection systems that can search for illicit radioactive sources in different environments.
Researchers from National Laboratories have been applying their expertise in radiation detection for more than 30 years. For example, detectors have been designed for use in treaty inspections can monitor the location of nuclear missile warheads in a nonintrusive manner. These detectors measure the gamma rays emitted from the isotopes of nuclear elements contained in weapons. Over the years, Laboratory researchers have developed a range of radiation detection instruments, including detectors on buoys for customs agents at U.S. maritime ports, cargo interrogation systems, and high-resolution handheld instruments that emergency response personnel could use to search for a clandestine nuclear device.
Gamma rays have the highest energy in the electromagnetic spectrum. They tend to go straight through matter, rather than reflect or bend as visible light does. Minors or lenses cannot be used to depict, or image, gamma rays, but their energy can be measured indirectly by observing how photons interact with a detector material. For many applications, however, researchers need to accurately determine where gamma rays originate, and doing so requires imaging technology. For example, many radiation detectors have excellent energy resolution and sensitivity to sources within a range of several meters. At greater distances, however, the source can be lost in a clutter of background gamma-ray emission from the environment, including concrete, natural mineral deposits, and some foods.
Detector developers want to design instruments that quickly survey large areas at a distance and accurately distinguish illicit from background signals. However, when a detector covers a large area, the signal from an object in the background can mimic the signature from a source of interest, even though the sources are widely separated. For example, a concrete building 20 meters from the detector may register the same as an illicit source located farther away. This similarity makes the detection of weak signals impossible unless the characteristics of the background are known in advance—unlikely in searches for clandestine radioactive materials.
Most state of the art gamma-ray imagers are collimator-based systems. They normally contain a collimating part that is made of a heavy, high Z material, such as lead or tungsten, and a position sensitive radiation detector. The imaging functionality relies on the fact that the collimator blocks the gamma-rays falling on it, casting a shadow on the detector surface. An image reconstruction algorithm analyzes the shadow, reconstructing the spatial distribution of tile radiation source. These gamma-ray imagers work well with low energy gamma-rays, but for gamma-rays of increasing energies, the collimators lose their absorbing efficiency, leading to low contrast, low sensitivity imaging.
Applications of gamma ray imaging such as those used in search and surveillance scenarios, as well as in applications which require mapping of radioactive sources distributed within a large field-of-view in the medium to large-field distances, require a different solution than the ones offered by standard tomographic methods.
Standard imaging systems are based on either collimator imaging or Compton scatter imaging, with collimator imaging more effective at lower gamma ray energies and Compton scatter imaging more efficient at higher gamma ray energies. Collimator-based imaging typically employs coded apertures for improved detector sensitivity. To date, there is no gamma ray detector arrangement which offers the high efficiency of coded apertures for low energy photons with the high efficiency and large field of view of Compton imagers for gamma rays, or photons, of higher energies for improved detection sensitivity and source location over the entire gamma ray spectrum.